1. Field of Invention
This invention relates to an improved switching power supply wherein an inductor device is provided to suppress a harmonic current therein, and wherein she power factor thereof is improved.
2. Description of the Prior Art
A conventional switching power supply is shown, for example, in FIG. 1, and comprises an AC power source AC connected to a diode bridge 11, consisting of diodes D1-D4, and a smoothing capacitor C1 connected in parallel to diode bridge 11. Diode bridge 11 is a rectifying circuit used to rectify the output of power source AC. The positive terminal of smoothing capacitor C1 is connected to terminal 21 of primary winding N1 of a transformer TR. The primary winding terminal 22 of winding N1 is connected to the negative terminal of smoothing capacitor C1 through switch Q. The terminals 23 and 24 of secondary winding N2 of transformer Tr are connected to rectifying circuit 25 consisting of diode 27 and smoothing capacitor C2. The output of rectifying circuit 25 is connected to a load circuit 31.
A control circuit 26 controls the output voltage of the rectifying circuit 25 and provides control signal GSW to switch Q so that the output voltage of rectifying circuit 25 is kept constant. Transformer TR, rectifying circuit 25, switch Q and control circuit 26 comprise a DC/DC converter 20 that performs an electrically isolated transfer of DC power, applied to the primary side of transformer TR to the secondary side thereof.
In switching power supply 10, a DC voltage VC1, obtained by AC/DC converting the output of AC power supply AC, by means of diode bridge 11 and smoothing capacitor C1, is applied to the terminal 21 of transformer TR of DC/DC converter 20, and is turned ON or OFF by switch Q, made for example, of a field effect transistor (FET). Hence, a switching current IN2 is induced in secondary winding N2 because current IN1 flows, as a train of pulses, through primary winding N1. Switching current IN2 is then converted to a DC current by rectifying circuit 25 which consists of diode D7 and smoothing capacitor C2, and is supplied to load circuit 31.
The switching power supply 10 AC/DC converts the output of AC power supply source AC, applied to rectifying circuit 11, using rectifying circuit 11 and smoothing capacitor C1. The DC power signal thus obtained is isolated by means of DC/DC converter 20, and then is supplied to load circuit 31. Accordingly, switching power supply 10 supplies a DC power signal which is isolated from the AC power source AC.
However, the input current ICI of smoothing capacitor C1 is not turned ON when the rectified voltage Vrec of AC power source AC is lower than voltage VC1, provided across smoothing capacitor C1. Hence, input current Iin (i.e. output current of AC power source AC) of switching power supply 10 takes the waveform of a pulse train, causing the conduction angle of input current Iin to become smaller. This results in decrease of the power factor and increase in harmonic current.
FIG. 2 shows waveforms of input voltage Vin and input current Iin of switching power supply 10, rectified voltage Vrect, and voltage VC1 provided across smoothing capacitor C1. As is evident from FIG. 2, input current Iin is not turned ON during interval T2, in which rectified voltage Vrec is lower than voltage VC1 provided across smoothing capacitor C1. In contrast, input current Iin is caused to flow as a series of pulses during interval T1, in which rectified voltage Vrec is higher than voltage VC1 provided across smoothing capacitor C1.
Accordingly, input current Iin flows as a series of pulses and causes the conduction angle of the input current Iin to become smaller. Thus, disadvantageously, in the conventional switching power supply, the power factor thereof is caused to decrease and the harmonic current is caused to increase.